1. Field of the Invention
The present invention relates to methods for making thin-film semiconductor devices and in particular to a method for making a thin-film semiconductor device, the method including an annealing step of crystallizing a semiconductor thin-film.
2. Description of the Related Art
In flat-panel displays, such as liquid crystal displays (LCDs) and organic electro-luminescence (EL) displays, thin-film transistors (TFTs) are used as the switching element for a plurality of pixels of active matrix displays. The TFTs are categorized into polycrystalline silicon TFTs having active regions composed of polycrystalline silicon (poly-Si) or microcrystalline silicon. (μc-Si) and amorphous silicon TFTs having active regions composed of amorphous silicon (amorphous Si). Polycrystalline silicon TFTs have a carrier mobility 10 to 100 times higher than that of the amorphous silicon TFTs and are thus excellent as the material of the switching elements. Accordingly, in the above-described flat-panel displays, it is highly desirable to use polycrystalline silicon TFTs as the switching elements in order to enhance the resolution of displayed images and to realize a system-on-panel technology, i.e., a technology of imparting various functions to the displays by providing various functions and circuits onto display substrates.
An example of the technique for fabricating polycrystalline silicon TFTs described above is a so-called low-temperature polysilicon process that uses only temperatures up to 600° C. The low-temperature polysilicon process is developed and already put to practical application. According to this process, it is no longer necessary to use expensive, though highly heat-resistant, substrates composed of quarts, single-crystal silicon, and the like, and high-resolution flat-panel displays can be fabricated at decreased costs.
Here, in the low-temperature silicon process, semiconductor thin films are formed by CVD involving decomposition by plasma and not heat. The resulting semiconductor thin film thus contains hydrogen. In the subsequent step, the semiconductor thin film is annealed at 450° C. or higher for several hours to remove hydrogen contained in the semiconductor thin film (dehydrogenation annealing). Subsequently, crystallization annealing by excimer laser irradiation is conducted to crystallize the semiconductor thin film. During the crystallization annealing, the large substrate is pulse-irradiated while shifting the region of the laser irradiation so that the substrate surface is uniformly irradiated with laser and that the size of the crystals can be made uniform over the entire surface of the substrate (refer to Japanese Unexamined Patent Application Publication No. 2000-340506, paragraph 0021).
As is described above, in the low-temperature polysilicon process, dehydrogenation annealing is performed ahead of the crystallization annealing using laser to prevent hydrogen ablation, i.e., a destruction of a film inflicted by evaporated hydrogen caused due to laser irradiation during the crystallization annealing.
Another example of the process involves gradually increasing the energy of laser irradiation to simultaneously perform dehydrogenation annealing and crystallization annealing. According to a report, the hydrogen content was reduced to the background level by increasing the energy of excimer laser irradiation in three steps, i.e., 150 mJ/cm2, 300 mJ/cm2, and 375 mJ/cm2 (refer to P. Mei, et al., Appl. Phys. Lett. (US) Feb. 28, 1994, vol 64(9), pp. 1132-1134).